Method and apparatus for automatic compensation of signal losses from transmission over conductors

ABSTRACT

A method and apparatus for automatic compensation of insertion loss in signals transmitted over conductors is presented. The present invention is particularly applicable to the transmission of signals over long lengths of CAT-5 or similar twisted-pair cables. A reference signal having a known form and strength (e.g. a pulse signal) is provided to each pair of conductors carrying a component of a signal from a transmitter to a receiver. The receiver includes adjustable gain amplifiers for each conductor pair over which a component of the signal is transmitted. The gains of the amplifiers are initially set at an initial level (e.g., their maximum gain) to allow detection of the reference signal. Once the reference signal is detected in a conductor pair, the amplifier gains are adjusted such that the level of the reference signal is restored approximately to its original form and strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/309,122, filed on Jun. 23, 2006, which is incorporated in itsentirety by reference herein.

FIELD OF THE INVENTION

This invention relates to the field of video transmission. Morespecifically the invention relates to compensation for losses in videosignals transmitted over conductors, including twisted pair conductors.

BACKGROUND OF THE INVENTION

Conductors (i.e. cables) are one method commonly used to conveyelectronic video signals from a source device (e.g., a video camera or aDVD player) to a destination device (e.g., a video display screen). Twotypes of cable commonly used for video transmission are coaxial cableand twisted pair cable. It is desirable for the video signal at thedestination device to correspond accurately to the original video signaltransmitted by the source device. “Insertion loss” is a term used todescribe signal degradation that occurs when a video or other signal istransmitted over a transmission medium such as a cable.

Typically, insertion loss is a function of the cable length: longerlength transmission cables will exhibit greater loss than shorter lengthcables. Coaxial cables typically exhibit less insertion loss thantwisted pair cables. However, coaxial cables are more expensive anddifficult to install than twisted pair cables. Twisted pair cablestypically are manufactured as bundles of several twisted pairs. Forexample, a common form of twisted pair cable known as “Category 5” or“CAT5” cable comprises four separate twisted pairs encased in a singlecable. CAT5 cable is typically terminated with an eight-pin RJ45connector.

Insertion loss is typically caused by the physical characteristics ofthe transmission cable. Insertion loss includes resistive losses (alsosometimes referred to as DC losses) as well as inductive, capacitive andskin effect losses (also sometimes referred to as AC losses). The ACinsertion loss exhibited by a cable is frequency dependent. For example,the insertion loss for a 1500 foot length of CAT5 cable as a function offrequency is shown in FIG. 11. In the example of FIG. 11, the insertionloss generally increases with increasing frequency, with the insertionloss for high frequency signals being significantly greater (−70 dB at50 MHz for a 1500 feet CAT-5 cable) than the DC insertion loss of 2.6 dBfor 1500 Feet (e.g. the loss at a frequency value of zero).

Video signals come in a variety of formats. Examples are CompositeVideo, S-Video, and YUV. Each format uses a color model for representingcolor information and a signal specification defining characteristics ofthe signals used to transmit the video information. For example, the“RGB” color model divides a color into red (R), green (G) and blue (B)components and transmits a separate signal for each color component.

In addition to color information, the video signal may also comprisehorizontal and vertical sync information needed at the destinationdevice to properly display the transmitted video signal. The horizontaland vertical sync signals may be transmitted over separate conductorsthan the video component signals. Alternatively, the sync signals may becombined with one or more of the video signal components and transmittedalong with those components.

For RGB video, several different formats exist for conveying horizontaland vertical sync information. These include RGBHV, RGBS, RGsB, andRsGsBs. In RGBHV, the horizontal and vertical sync signals are eachcarried on separate conductors. Thus, five conductors are used: one foreach of the red component, the green component, the blue component, thehorizontal sync signal, and the vertical sync signal. In RGBS, thehorizontal and vertical sync signals are combined into a composite syncsignal and sent on a single conductor. In RGsB, the composite syncsignal is combined with the green component. This combination ispossible because the sync signals comprise pulses that are sent during ablanking interval, when no video signals are present. In RsGsBs, thecomposite sync signal is combined with each of the red, green and bluecomponents. Prior art devices exist for converting from one format ofRGB to another. To reduce cabling requirements, for transmission of RGBvideo over anything other than short distances, a format in which thesync signals are combined with one or more of the color componentsignals is commonly used.

Although twisted pair cables are convenient and economical fortransmission of video signals, signal degradation (skew between videosignal components and insertion loss) limits the distance over whichsatisfactory quality video signals can be transmitted via twisted paircables. Video transmitter/receiver systems exist that amplify videosignals transmitted over twisted-pair cables. In such systems, atransmitter amplifies the video source signal prior to being transmittedover twisted pair cable, and a receiver amplifies the received signal.These transmitter/receiver systems allow longer transmission distancesover twisted-pair cable than are possible for unamplified signals.However, to prevent signal distortion, the amount of gain(amplification) supplied by the transmitter and receiver must beproperly matched to the amount of insertion loss that occurs in thelength of the twisted-pair cable over which the video signal istransmitted. If the gain applied it is too high, clipping will occur. Ifthe gain is too low, low-level portions of the original input signal maybe lost. Ideally the system gain should be flat across the frequencyspectrum. High frequency loss results in smearing and loss of focus inthe video.

There exists a need for a video transmission system that automaticallycompensates for signal losses that occur for video signals transmittedover appreciable distances via conductors, including twisted paircables.

SUMMARY OF THE INVENTION

The invention comprises a method and apparatus for automaticcompensation of insertion loss in video signals transmitted overconductors. The present invention is particularly applicable to thetransmission of video over long lengths of CAT-5 or similar twisted-paircables. Embodiments of the invention may be implemented as a separatedevice and/or as part of a video transmission system.

In one or more embodiments, a reference signal having a known form andstrength (e.g. a pulse signal) is provided to each pair of conductorscarrying a component of a video signal from a transmitter to a receiver.The reference signal, for instance, comprise a modified or altered formof a sync signal of the input video signal (e.g. the horizontal orvertical sync signals). The receiver includes adjustable gain amplifiersfor each conductor pair over which a component of the video signal istransmitted. The gains of the amplifiers are initially set at an initiallevel (e.g., their maximum gain) to allow detection of the referencesignal in each pair of conductors. Once the reference signal is detectedin a conductor pair, the amplifier gains are adjusted such that thelevel of the reference signal is restored approximately to its originalvalue. In one or more embodiments, the amplifiers are configured toexhibit a frequency response that is the inverse to and compensates forthe frequency response loss in the cable over which the video signal isbeing transmitted.

Further objects, features, and advantages of the present invention overthe prior art will become apparent from the detailed description of thedrawings which follows, when considered with the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of long distance twisted pair transmissionapparatus in accordance with an embodiment of the present invention.

FIG. 2 is an illustration of allocation of the conductors of twistedpair cable 106 for various video formats in accordance with anembodiment of the present invention.

FIG. 3 is an illustration of allocation of the conductors of twistedpair cable 106 for Video in accordance with an embodiment of the presentinvention.

FIG. 4 is a block diagram illustration of architecture of transmitter104 in accordance with an embodiment of the present invention.

FIG. 5 is an illustration of a polarity converter in accordance with anembodiment of the present invention.

FIG. 6 is a block diagram illustration of architecture of Receiver 108in accordance with an embodiment of the present invention.

FIG. 7 is an illustration of a sync stripper circuit in accordance withan embodiment of the present invention.

FIG. 8 is an illustration of a compensation circuit in accordance withan embodiment of the present invention.

FIG. 9A is an illustration of a variable AC and DC compensation circuitin accordance with an embodiment of the present invention.

FIG. 9B is an illustration of a Fixed AC and DC compensation circuit inaccordance with an embodiment of the present invention.

FIG. 10 is an illustration of circuit for determination of the fine andcoarse gain selection signals in accordance with an embodiment of thepresent invention.

FIG. 11 is a frequency response plot of an example 1500 feet length CAT5cable.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises a method and apparatus for compensation ofinsertion loss in video signals transmitted over conductors, such astwisted pair cable. In the following description, numerous specificdetails are set forth to provide a thorough description of theinvention. It will be apparent, however, to one skilled in the art, thatthe invention may be practiced without these specific details. In otherinstances, well-known features have not been described in detail so asnot to obscure the invention.

In one or more embodiments, the invention comprises a transmitter and areceiver that enable communication of video signals, e.g. compositevideo, S-Video, computer-video, and other high resolution video, overextended distances of conductors, including, for example CAT 5 orsimilar twisted-pair cables.

In one or more embodiments, restoration of the video signal received bythe receiver is accomplished using a reference signal. In one or moreembodiments, the reference signal is a periodic pulse signal added bythe transmitter to the video signal during blanking intervals of thevideo signal. In one or more embodiments, a modified (standardized)horizontal or vertical sync signal generated from the input video signalis used as the reference signal. The reference signal is added by thetransmitter to each video component signal. When the receiver is coupledto the transmitter via a cable, the receiver initially detects thepresence of the reference signal in the video component signals. In oneor more embodiments, the receiver includes variable gain amplifiers foreach signal component that are adjusted to an initial level fordetection of the signal. In one or more embodiments, the initial levelis the maximum gain of the amplifiers. In these embodiments, theamplifier gain for a video signal component remains at its maximum untilthe presence of the reference signal in that signal component has beendetected. Once the reference signal is detected, the amplifier gains areadjusted until the reference signal is restored to a desired level, forexample, the level of the original reference signal. In one or moreembodiments, the amplifiers are configured to exhibit a frequencyresponse that is the inverse to and compensates for the frequencyresponse loss due to the cable over which the video signal is beingtransmitted.

An embodiment of a video transmission system that implements the presentinvention is illustrated in FIG. 1. The video transmission system ofFIG. 1 comprises video source 102, cable 103, transmitter 104; twistedpair cable 106; receiver 108, cable 109 and destination device 110.Cable 103 couples the video (and audio, if applicable) signals fromsource 102 to transmitter 104. Cable 103 may comprise any suitableconductors known in the art for coupling the type of video signalgenerated by video source 102 to transmitter 104. Transmitter 104comprises multiple input terminals for accepting different input signalformats. For example, transmitter 104 may comprise connectors foraccepting a composite video signal, an S-Video signal, a digital videosignal, an RGB component video signal, etc. Transmitter 104 may alsocomprise standard audio connectors such as, for example RCA input jacks.

In one or more embodiments, cable 106 comprises a cable comprising abundle of multiple twisted pair conductors. For example, cable 106 maycomprise a CAT5 or similar cable comprising four pairs of twistedconductors and terminated with standard male RJ-45 connectors that matewith matching female RJ-45 connectors on the transmitter and receiver,respectively. The pairs of twisted conductors may, for example, beallocated as shown in FIGS. 2 and 3.

Example embodiments of the present invention are described using RGBHVas an example video input signal format and CAT5 cable as an exampleconductor over which the video signal is transmitted. However, it willbe clear to those of skill in the art that the invention is not limitedto RGBHV and CAT5 cable and other video formats and other cable typesmay be used.

FIG. 4 is a block diagram showing the architecture of transmitter 104 ofFIG. 1 in an embodiment of the present invention. In the embodimentshown in FIG. 4, transmitter 104 receives a video source signalcomprising separate video input signals and sync input signals. Forexample, if the video input source signal is in RGBHV format, the videoinput signals comprise the R, G and B signals, while the sync inputsignals comprise the H and V sync signals. In other embodiments, thesync signals may be combined with one or more of the video componentsignals.

In embodiments using video formats in which the sync information iscombined with one or more of the video component signals (e.g. S-Video,Component video, or RGB video with a combined synchronization signal),the sync signals may be detected and extracted from one or more of theinput video component signals and then re-combined with one or more ofthe video components to provide reference signals for signalcompensation as well as providing sync information.

In the RGBHV embodiment of FIG. 4, transmitter 104 comprises horizontaland vertical sync input terminals 431H and 431V, red, green and bluevideo input terminals 401R, 401G and 401B, input amplifiers 410R, 410G,and 410B, back porch clamp (BPC) generator 430, offset correctioncircuits 440R, 440G, and 440B, uni-polar pulse converters 450H and 450V,differential output amplifiers 460R, 460G and 460B, and differentialoutput terminals 402R, 402G and 402B. Transmitter 104 may also containlocal output amplifiers for each input signal (not shown) that provide alocal video monitor output signal.

Input amplifiers 410 receive the input video signal from video inputterminals 401, and uni-polar pulse converters 450 receive the sync inputsignals from sync input terminals 431. In one or more embodiments,separate amplifiers are utilized for each video component signal. Forexample, in an embodiment for an RGBHV input signal, three inputamplifiers 410 for the video components (one each for the R, G, and Bcomponents) and two uni-polar pulse converters 450 for the sync signals(one each for the H and V sync signals) are used.

Input amplifiers 410 are used in conjunction with horizontal sync BPCgenerator 430 and offset correction circuits 440 to detect andcompensate for any DC offset in the source video signal. In theembodiment of FIG. 4, offset correction circuits 440 determine the DCoffset for each video component using the back porch clamp signal fromthe BPC generator 430, and the amplified video source signal from inputamplifiers 410. Offset correction circuits 440 apply compensation toeach video component via a feedback loop comprising the respective inputamplifier 410 for that component.

The vertical and horizontal synchronization signals 431H and 431V arecoupled to uni-polar pulse converters 450. Uni-polar pulse converters450 assure that sync signals output by transmitter 104 are always thesame polarity regardless of the polarity of the input. An embodiment ofa uni-polar pulse converter 450 is illustrated in FIG. 5.

In the embodiment of FIG. 5, pulse converter 450 comprises twoexclusive-OR gates (e.g. 510 and 520) that process the received syncinput signal. Initially, the sync input signal 501 (e.g. 431H and 431V)is exclusive-ORed with ground in gate 510 and then the output of gate510 is filtered in low-pass filter 530 (which in one or more embodimentscomprise a resistor and capacitor circuit) and exclusive-ORed withitself (i.e. unfiltered output of gate 510) in gate 520 to generate thepolarity-corrected sync output signal 502.

In one or more embodiments, the standardized horizontal sync signal(identified as “H_(SYNCP)” in FIG. 4) is used as both the horizontalsync signal and as the reference pulse signal. In these embodiments,H_(SYNCP) is injected into each of the video signal componentssimultaneously. In addition, the vertical sync signal (“V_(SYNCP)” inFIG. 4) is added to one of the video components to convey the verticalsync information to the receiver.

In the embodiment of FIG. 4, the red video component signal is used toconvey the vertical sync information. H_(SYNCP) is summed with V_(SYNCP)at node 452 and subtracted from the red video component signal (added tothe negative input terminal) at differential amplifier 460R. H_(SYNCP)is subtracted from the green video component at differential amplifier460G; and H_(SYNCP) is subtracted from the blue video component atdifferential amplifier 460B. In this way, a negative reference pulse issimultaneously added to all three differential video output signals.

Differential output amplifiers 460 receive the reference, sync (ifapplicable) and video signals and provide corresponding amplifieddifferential driver signals to differential output terminals 402. In oneor more embodiments, differential output terminals 402 comprise a femaleRJ-45 connector using pin assignments such as those shown in FIG. 2(pins 3 and 6 may be used for transmission of power, digital signals,and/or audio signals). Differential output terminals 402 may beconnected via twisted pair cable 106 of FIG. 1 to receiver 108.

Receiver 108 receives the differential video signals from transmitter104 via twisted pair cable 106. Receiver 108 processes the differentialvideo signals to compensate for insertion loss (and, in one or moreembodiments, for skew) and outputs the compensated video signals to adestination device such as projector 110. FIG. 6 is a block diagram ofreceiver 108 in accordance with an embodiment of the present invention.

In the embodiment of FIG. 6, Receiver 108 comprises variable gainamplifiers 610R, 610G and 610B, discrete gain amplifiers 620R, 620G and620B, skew adjustment circuit 630; output stages 640R, 640G and 640B, DCoffset compensation circuits 622R, 622B and 622G, and sync detectors650H and 650V. Receiver 108 may also include differential outputterminals (not shown) that output a buffered and/or amplified version ofthe input signals for daisy chaining to other receivers.

The differential video input signals 601 (e.g. 601R, 601G and 601B) arecoupled to the respective variable gain amplifiers 610 and discrete gainamplifiers 620. Each variable gain amplifier 610 works together with thecorresponding discrete gain amplifier 620 to compensate a respective oneof the differential input video signals for insertion losses resultingfrom communication of the signal from transmitter 104 to receiver 108over twisted pair cable 106. In one or more embodiments, each variablegain amplifier 610 is capable of providing a controllable, variableamount of gain over a range from zero (0) to a maximum value (K), andeach discrete gain amplifier 620 provides amplification in controllable,discrete multiples of K (e.g. 0K, 1K, 2K, etc). Together, variable gainamplifiers 610 and discrete gain amplifiers 620 provide controllableamounts of variable gain over an amplification range equal to the sum ofthe maximum gain of variable gain amplifiers 610 and the maximum gain ofdiscrete gain amplifiers 620. In one or more embodiments, K representsthe amount of gain typically required to compensate for signal lossesover a known length of cable (e.g. 300 feet).

The total amount of gain provided by variable gain amplifiers 610 anddiscrete gain amplifiers 620 may be selected based on the length ofcable 106, or may be automatically controlled. The amount of gainprovided by variable gain amplifiers 610 and discrete gain amplifiers620 may be controlled, for example, using a micro-controller thatdetermines the appropriate amount of gain to be applied based on actualand expected signal strength of the reference signal included in thevideo signals received from transmitter 104, as is described in greaterdetail below.

FIG. 8 shows embodiments of a variable gain amplifier 610 and a discretegain amplifier 620 in one embodiment of the invention. FIG. 8 shows avariable gain amplifier 610 and discrete gain amplifier 620 for a singlevideo signal component, namely the red color component of an RGB signal(designated R_(x) in FIG. 8). However, it will be understood that in oneor more embodiments each color component is provided with its ownvariable gain amplifier 610 and discrete gain amplifier 620, as shown,for example, in FIG. 6.

In the embodiment of FIG. 8, variable gain amplifier 610 providesamplification over an initial amplification range of zero up to amaximum gain (represented herein by the letter “K”). Discrete gainamplifier 620 provides selectable, discrete amounts of gain in multiplesof K. For example, in the embodiment of FIG. 8, discrete gain amplifier620 provides selectable gain in the amounts of 0K, 1K, 2K, 3K or 4K.Together, variable gain amplifier 610 and discrete gain amplifier 620provide continuously variable gain over the range from 0 to 5K.

In the embodiment of FIG. 8, variable gain amplifier 610 includes afixed gain amplifier circuit (FGA) 850, a variable gain amplifiercircuit (VGA) 840, and a compensation circuit 842. VGA 840 and FGA 850are both coupled to the differential input signals R_(x)(+ve) 801P andR_(x)(−ve) 801N. The coupling may be via a differential line buffer,e.g. 810, to prevent unbalancing of the transmission line. FGA 850converts the differential video input signal to a single ended outputwith fixed gain. VGA 840 adds a controllable amount of variable (DC andAC Compensation) gain to the differential video input signal. Theoutputs of FGA 850 and VGA 840 are summed at node 843. The resultingsummed signal is provided to the input of discrete gain amplifier 620from node 845.

The amount of gain supplied by VGA 840 is controlled by Fine GainControl Signal 805 supplied, for example, by a microcontroller.Compensator circuit 842 is used to set the desired frequency response ofVGA 840. An embodiment of compensator circuit 842 is illustrated in FIG.9A. The fine gain control of VGA 840 compensates for both DC and ACsignal losses in cable lengths of 0 feet to N feet (e.g. 300 feet).

FIG. 9A shows an embodiment in which compensator circuit 842 isconnected to voltage controlled amplifier 960. In the embodiment of FIG.9A, compensator circuit 842 is connected to the gain setting inputs ofthe voltage controlled gain amplifier 960. Compensator circuit 842comprises a gain setting resistor 950 and one or more series-connectedresistor/capacitor pairs (i.e. 910, 920, 930, and 940) connected to thegain setting inputs of amplifier 960. Gain setting resistor 950 sets theDC gain compensation for the amplifier, while resistor/capacitor pairs910-940 set the poles (AC gain), thereby shaping the transfer functionof the amplifier circuit. Although four pole-setting resistor/capacitorpairs are used in the embodiment of FIG. 9, a greater or lesser numbermay be used depending on the shape of the transfer function desired. Inone or more embodiments, the values of the resistors and capacitors arechosen such that the frequency response of the amplifier correspondsinversely to the frequency response of the cable over which the videosignals are being transmitted, and such that the overall maximum dB gain(e.g. “K”), when the fine gain control is set to maximum, compensatesfor signal loss expected for a predetermined length of the type of cableused (e.g. 300 feet of CAT5 cable). The value (and frequency response)of “K” may be determined by measurement or may be estimated based on theknown characteristics of the cable, such as, for example, the frequencyresponse of CAT5 cable shown in FIG. 11.

Referring back to FIG. 8, if the maximum gain “K” provided by variablegain amplifier 610 corresponds to the insertion loss exhibited by 300feet of CAT5 cable, then variable gain amplifier 610 can providevariable signal compensation corresponding to 0 to 300 feet of CAT5cable. The amount of gain, between 0 and K (e.g. for between 0 and 300foot lengths of CAT5 cable) provided by variable gain amplifier 610 iscontrolled by fine gain control signal 805 (described in greater detailbelow). For longer lengths of cable, additional signal amplification isrequired. In the embodiment of FIG. 8, that additional signalamplification is provided by discrete gain amplifier 620.

Discrete gain amplifier 620 provides additional compensation for longerline lengths in discrete amounts of “K”. For example, for a cable lengthof 450 feet, 1.5K of total compensation is required. In this case,discrete gain amplifier 620 provides 1K (300 feet) of compensation,while variable gain amplifier 610 provides the remaining 0.5K (150 feet)of compensation.

In the embodiment of FIG. 8, discrete gain amplifier 620 comprises amultiplexer 820, a zero-gain buffer 803, and a plurality of fixed gaincompensation circuits 806, 809, 812 and 815. Each compensation circuitprovides a fixed amount of gain (e.g. “K”) that is approximately equalto the maximum amount of gain provided by a variable gain amplifier 610,and that has approximately the same frequency response. FIG. 9B showsthe fixed compensation network encompassed in the fixed gain stages 806,809, 812, and 815 of FIG. 8. The compensation network is placed in thefeedback loop of amplifier 982. The fixed gain compensation networkcomprises a gain setting resistor 980 and one or more series-connectedresistor/capacitor pairs (i.e. 972, 974, 976, and 978) connected inparallel between the inverting input of amplifier 982 and ground. Gainsetting resistor 980 sets the fixed DC gain, while theresistor/capacitor pairs set the poles, thereby shaping the transferfunction of the amplifier circuit for a fixed amount of gaincompensation (e.g. “K”). The compensation circuits are daisy changed,and the output of each successive compensation circuit is connected toone of the inputs of multiplexer 820.

In one or more embodiments, the amplifier circuit of FIG. 9B is used foreach of the compensation circuits 806, 809, 812 and 815.

In the embodiment of FIG. 8, input 831 of multiplexer 820 is connectedto the output of buffer 803 (i.e. the buffered output signal fromvariable gain amplifier 610). Input 832 is connected to the output ofcompensation circuit 806 (i.e. the output signal from variable gainamplifier 610 after it has been amplified by compensation circuit 806).Input 833 is connected to the output of compensation circuit 809 (i.e.the output signal from variable gain amplifier 610 after having beenamplified by compensation circuits 806 and 809). Input 834 is connectedto the output of compensation circuit 812 (i.e. the output signal fromvariable gain amplifier 610 after having been amplified by compensationcircuits 806, 809 and 812). Input 835 is connected to the output ofcompensation circuit 815 (i.e. the output signal from variable gainamplifier 610 after having been amplified by compensation circuits 806,809, 812 and 815). If K is the amount of gain provided by eachcompensation circuit, then the additional gain applied to the outputsignal from variable gain amplifier 610 is 0K, 1K, 2K, 3K or 4K,depending on which of inputs 831, 832, 833, 834 or 835 is selected. Ifthe amount of gain supplied by variable gain amplifier 610 is “J” (i.e.a value between 0 and K), the total amount of gain provided by variablegain amplifier 610 and discrete gain amplifier 620 is J, J+K, J+2K, J+3Kor J+4K, depending on which of inputs 831, 832, 833, 834 or 835 isselected.

In the embodiment of FIG. 8, the fixed amount of compensation providedby each of compensation of circuits 806, 809, 812 and 815 isapproximately equal to the maximum compensation provided by variablegain amplifier 610. However, it will be obvious to those of skill in theart that the amount of compensation provided by each of the compensationcircuits 806, 809, 812 and 815 may be greater or less than the maximumprovided by variable gain amplifier 610. Further, the discrete amount ofcompensation provided by each of compensation circuits 806, 809, 812 and815 need not be the same.

The connection of either of inputs 831, 832, 833, 834 or 835 to output802 of multiplexer 820 is controlled by coarse gain selection signal807. In one or more embodiments, coarse gain selection signal 807 isgenerated by a micro-controller, which determines both the coarse gainselection signal 807 and the fine gain control signal 805 based on theactual loss in the reference signal as detected in the video signalreceived from the transmitter as described with respect to FIG. 10below.

If “K” is the amount of compensation corresponding to the losses (in dB)exhibited by 300 feet of CAT5 cable, if maximum compensation provided byvariable gain amplifier 610 is “K”, and if each compensation circuit806, 809, 812 and 815 provides a fixed amount “K” of compensation, thenthe embodiment of FIG. 8 provides variable compensation to compensatefor insertion loss for video transmitted over between zero (0) andfifteen hundred (1500) feet of CAT5 cable.

FIG. 10 shows a circuit for determination of the fine and coarse gainselection signals in accordance with an embodiment of the invention.Gain selection involves measuring the loss in the reference signal addedby the transmitter to the video component signals transmitted to thereceiver. In one or more embodiments, the loss exhibited by thereference signal in one of video component signals received by thereceiver is determined, and the appropriate amount of compensationneeded to restore the reference signal to its original level iscalculated. That amount of compensation is then applied to all colorcomponent signals. In other embodiments, the amount of loss, andcorresponding amount of compensation, may be separately determined foreach video signal component.

In the embodiment of FIG. 10, level comparator 1018 is used inconjunction with sample timing generator 1030, sampler 1060, andmicrocontroller 1014 to determine the amount of compensation needed torestore the reference signal to the desired value. Sampler 1060 samplesthe reference signal at the receiver (sync reference pulse at input ofOutput Amplifier 1040) for a short duration (e.g. 50 ns) during theleading (falling) edge of the reference signal using gating informationprovided by sample Timing Generator 1030. Comparator 1018 receives thesampled signal 1002 at its positive input terminal, and compares it to anegative reference voltage level (−V_(Ref)) 1003 coupled to its negativeinput terminal. This comparison generates the differences for the DClevel and AC step responses. The output of comparator 1018 is a TTLlevel signal that connects to the clock input of a D-type register 1016.Microcontroller 1014 resets D-type register 1016 and monitors the output(i.e. Q) of the register 1016. If the level of the output signal ofcomparator 1018 is sufficient, it will clock register 1016, indicatingto microcontroller 1014 that the level of the reference signal is at orgreater than the desired (approx. original) level. If the output ofcomparator 1018 is not sufficient to clock register 1016,microcontroller 1014 commands discrete gain amplifier 620 and variablegain amplifier 610 to provide an increased amount of compensation to thevideo component signal. If the output of comparator 1018 is sufficientto clock register 1016, in one or more embodiments, microcontroller 1014commands discrete gain amplifier 620 and variable gain amplifier 610 toprovide incrementally reduced amounts of compensation until the point isfound where the output of comparator 1018 is just sufficient to clockregister 1016.

In the embodiment of FIG. 10, microcontroller 1014 may use a softwarealgorithm to determine the total amount of compensation to be applied,and then determine the number of compensation circuits to be used indiscrete gain amplifier 620 and the amount of additional compensation tobe supplied by variable gain amplifier 610 to obtain the desired totalamount of compensation. Microcontroller sends a corresponding coarsegain selection signal 807 to discrete gain amplifier 620, and commandsdigital potentiometer 1012 to provide the corresponding fine gaincontrol signal to variable gain amplifier 610. In one or moreembodiments, microcontroller 1014 initially sets the gain to an initiallevel (which may be the maximum gain), and increases or decreases ituntil the reference signal is restored, which is indicated by comparator1018 clocking the D-type register 1016. In one or more embodiments, thereference signal is continuously monitored to determine when signalcompensation is required. Thus, if the cable over which the video signalis transmitted from the transmitter to the receiver is changed, switchedor the insertion loss changes for any other reason, the insertion losswill be compensated for automatically.

Referring back to FIG. 6, after the video components are compensated forinsertion loss, the video signals may be further compensated for skewusing skew adjustment circuit 630, and for DC offset using DC OffsetCompensation circuit 622.

In one or more embodiments, skew adjustment may be accomplished bydetecting the reference signal in each video output component,determining the amount of skew delay among the video signal components,and determining the amount of delay to be applied to each component toeliminate any detected skew. An example skew adjustment circuit isdescribed in co-pending U.S. patent application Ser. No. 11/309,120,entitled “Method And Apparatus For Automatic Compensation Of Skew InVideo Transmitted Over Multiple Conductors”, the specification of whichis incorporated by reference herein.

DC offset compensation may be applied using circuit 622 (i.e. 622B, 622Gor 622R) which comprises a feedback loop around the skew adjustmentcircuit 630 and output amplifiers 640. The DC offset compensationcircuit 622 for each respective color component signal measures thesignal offset (from ground) at the respective output of the video outputamplifiers 640, and generates a corresponding correction signal,accounting for DC offset caused by the receiver's circuitry itself. Inthe embodiment of FIG. 6, each DC offset correction signal feeds backand sums at the respective summing node 624 with the respective gaincompensated video signal component (from the respective discrete gainamplifier 620).

Output signals 602R, 602G and 602B are generated by strippingappropriate reference and/or sync signals from the video signalcomponents by respective output stages 640R, 640G and 640B. In one ormore embodiments, an output stage 640 comprises a switch that groundsthe video output during the sync period. When either the vertical sync(e.g. 603V) or the horizontal sync (e.g. 603H) pulse is high for anyvideo component signal, the video output (i.e. 602) is switched toground; otherwise, the video output is switched to the correspondingvideo signal output of skew adjustment circuit 630. An embodiment of aswitch arrangement that is used in the present invention is illustratedin FIG. 7. In other non-RGBHV video formats the sync signals may not bestripped and/or sync signals may be added to certain video components tore-constitute the video format being transmitted.

In FIG. 7, R_(x) 701 is the video source from the output of skewadjustment circuit 630, and R_(y) 702 is the stripped video output. Thevertical sync strip signal (i.e. V_(Sync)) is OR'd with the horizontalsync strip signal (i.e. H_(Sync)) to generate the “Select” signal ofswitch 710. In one or more embodiments, the vertical and horizontal syncstrip signals are provided to output stages 640 by circuitry thatdetects the sync signals from sync detectors 650H and 650V (shown inFIG. 6). When the Select signal is true (“T”) the video output, R_(y)702, is coupled to ground through switch 710 to remove the sync pulse.Otherwise, i.e. when the Select signal is false (“F”), the video outputR_(y) 702 is coupled to the input signal, R_(x) 701.

In the embodiment of FIG. 6, the sync pulses are detected by comparingthe appropriate color component signal (e.g. the Red (i.e. R_(y))component for the vertical sync signal and the Blue (i.e. B_(y))component for the horizontal sync signal) at the corresponding output ofskew adjustment circuit 630 against appropriate reference voltagelevels. A comparator may be used for such comparison. Thus, in theembodiment of FIG. 6, the vertical sync signal is generated by verticalsync detector 650V when the R_(y) output of skew adjustment circuit 630meets the reference voltage threshold level, V_(REF), and the horizontalsync signal is generated by horizontal sync detector 650H when the B_(y)output of skew adjustment circuit 630 meets the reference voltagethreshold level, H_(REF).

Thus, a novel compensation method and system for automatic compensationof insertion loss in video transmitted over conductors has beendescribed. It will be understood that the above described arrangementsof apparatus and methods are merely illustrative of applications of theprinciples of this invention and many other embodiments andmodifications may be made without departing from the spirit and scope ofthe invention as defined in the claims. For example, although exampleembodiments have been described for video signals that comprise threecolor components transmitted over twisted pair conductors, the inventioncan be used with any type of signal that is transmitted over any numberor type of conductors, as will be understood by those of skill in theart.

The invention claimed is:
 1. A method for automatic compensation oflosses in a signal transmitted over conductors and received by areceiving apparatus comprising: receiving by said receiving apparatus ofan input signal over a conductor, said input signal comprising areference signal; generating by said receiving apparatus an outputsignal from said input signal by applying compensation to said inputsignal, wherein said applying said compensation comprises: applying aninitial amount of compensation by compensation circuitry of saidreceiving apparatus, said compensation circuitry configured to produce avariable amount of compensation up to a maximum amount of compensation,wherein said initial amount of compensation comprises said maximumamount of compensation; measuring a current level of said referencesignal in said output signal; comparing said current level of saidreference signal to a first level; and adjusting an amount ofcompensation applied by said compensation circuitry to said input signaluntil said current level of said reference signal is approximately equalto said first level.
 2. The method of claim 1, wherein said referencesignal comprises a negative voltage pulse.
 3. The method of claim 1,wherein said reference signal is injected into said input signal at atransmitting station prior to said input signal to being received bysaid receiving apparatus.
 4. The method of claim 1, wherein saidmeasuring of said current level of said reference signal comprises:sampling said output signal at a leading edge of said reference signal.5. The method of claim 1, wherein said input signal comprises is acomponent of a multi-component signal.
 6. The method of claim 1, whereinsaid input signal comprises a differential signal.
 7. The method ofclaim 1, wherein said compensation circuitry comprises a plurality offixed compensation circuits coupled to a variable compensation circuit.8. The method of claim 7, wherein said step of adjusting said amount ofcompensation applied to said input signal by said compensation circuitrycomprises: adjusting a compensation level of said variable compensationcircuit; and selecting a number of said plurality of fixed compensationcircuits.
 9. The method of claim 8, wherein said variable compensationcircuit is configured to provide a variable amount of compensation up toa first compensation level.
 10. The method of claim 9, wherein each ofsaid plurality of fixed compensation circuits is configured to provide afixed amount of compensation approximately equal to said firstcompensation level.
 11. An apparatus for compensating for losses insignals transmitted over conductors comprising: an input configured toreceive an input signal comprising a reference signal transmitted over aconductor; a compensation circuit coupled to said input configured togenerate an output signal by providing an amount of compensation up to amaximum amount of compensation to said input signal, wherein saidcompensation circuit is further configured to be controlled by one ormore control signals; and a controller circuit configured to generateone or more of said control signals depending on a sampled value of saidreference signal, said controller circuit configured to control saidcompensation circuit to provide said maximum amount of compensation tosaid input signal when said input signal is received at said input. 12.The apparatus of claim 11, wherein said reference signal comprises anegative voltage pulse.
 13. The apparatus of claim 11, wherein saidinput signal comprises a video signal.
 14. The apparatus of claim 11,wherein said controller circuit comprises: a comparator coupled to saidoutput signal configured to generate a first signal when said sampledvalue is approximately equal to a first value; and a microcontrollercoupled to said comparator and said compensation circuit, wherein saidmicrocontroller is configured to generate said one or more of saidcontrol signals to drive said sampled value to approximately said firstvalue.
 15. The apparatus of claim 11, wherein said input signalcomprises a component of a multi-component signal.
 16. The apparatus ofclaim 11, wherein said input signal comprises a differential signal. 17.The apparatus of claim 11, wherein said compensation circuit comprises aplurality of fixed compensation circuits coupled to a variablecompensation circuit.
 18. The apparatus of claim 17, wherein saidvariable compensation circuit is configured to provide a variable amountof compensation up to a first compensation level.
 19. The apparatus ofclaim 18, wherein each of said plurality of fixed compensation circuitsis configured to provide a fixed amount of compensation approximatelyequal to said first compensation level.
 20. The apparatus of claim 11wherein said reference signal comprises a sync signal.